1. Field of the Invention
The present invention is broadly concerned with improved methods for forming superconducting materials and thermoelectric materials comprising highly volatile elements. In one embodiment, the methods are useful for the production of highly epitaxial Hg-containing film superconductors exhibiting very high Jc values, along with novel films of this character. In another embodiment, the methods are useful for the production of thermoelectric materials which have very low thermal conductivities. In either embodiment, a process is carried out whereby precursor structures including a nonvolatile element are subjected to energy in the presence of a vapor comprising a volatile element so as to cause the nonvolatile element to be replaced by the volatile element without substantial alteration of the crystalline structure of the precursor.
2. Description of the Prior Art
Hg-based superconductors have a record high superconducting transition temperature (Tcxcx9c135K). Since a higher Tc promises higher device operation temperature and a better stability at a given temperature, it is very important to develop viable technologies for fabrication of high-quality Hg-based superconducting films. Prior technologies generally involve two steps: deposition of amorphous rare-earth cuprate precursor films with or without Hg, followed by Hg-vapor annealing at temperatures above 800xc2x0 C. under a controlled Hg-vapor pressure in order to form the superconducting phases.
All high Tc superconductors have layered structures and their physical properties are anisotropic parallel or perpendicular to the layers. The alignment of grains during the growth is crucial for the quality of the films. Since most applications of the superconductor require the capability of carrying high current, epitaxial growth of grains is essential.
Since Hg-based compounds are volatile, the control of the growth conditions is very difficult using conventional techniques. Though c-axis-oriented superconducting Hg-1212 and Hg-1223 films have been obtained with their Tc up to 124K and 130K, respectively, high-quality epitaxial growth has not been achieved. In other words, the superconducting grains are connected more or less randomly in the plane of the substrates. This lack of epitaxy is reflected in the poor x-ray pole figures and high Xmin values of prior Hg-1212 and Hg-1223 films, which are on the order of 100%.
A direct effect of this substantial non-epitaxy is that the Jc values of the films are much lower than expected in view of the intragrain Jc values and the high irreversibility line of the Hg-based superconductors. When current passes grain boundaries, it is significantly reduced owing to the fact that the grains are not aligned epitaxially. The other effect of this non-epitaxy is a relatively rough film surface which hinders many potential applications of the Hg-based superconducting thin films, e.g., for use in microelectronics.
The volatility of Hg presents a particular problem in the fabrication of micro-bridges in microelectronic devices. Because Hg-based superconductors are so delicate and tend to react with etching chemicals and water, fabrication of the Hg-containing micro-bridges directly from Hg-containing films using regular photolithography processes generally results in degradation of the samples. This, in turn, leads to unreliable connections in the circuit.
There is accordingly a real and unsatisfied need in the art for an improved method of fabricating Hg-containing superconductors to yield films having a high degree of epitaxy and correspondingly high Tc and Jc values.
The properties of thermoelectric materials are demonstrated by calculating the figure of merit ZT=TS2/xcexaxcfx81, where T is the operating temperature of the material, S is the Seebeck coefficient, xcfx81 is the resistivity, and K is the thermal conductivity of the material. Current state-of-the-art Peltier refrigerators use semiconducting Bi2Te3xe2x80x94Sb2Te3 alloys (with a ZT of  less than 1) that only produce moderate amounts of cooling and are inefficient compared to compressor-based refrigerators. As a result, thermoelectric refrigerators are generally used in applications where reliability and convenience are more important than economy.
Many attempts have been made to design improved thermoelectric materials having a higher ZT. Theoretically, a solid that is simultaneously a poor conductor of heat and a good conductor of electricity (the xe2x80x9celectron crystal and phonon glassxe2x80x9d) could have a ZT as high as 2-4, making it ideal for thermoelectric applications.
It has been proposed that synthesizing semiconducting compounds in which one of the atoms or molecules is weakly bound in an oversized atomic cage would cause the weakly bound atom to undergo local anharmonic vibrations, somewhat independent of the other atoms in the crystal, forming what is known as a xe2x80x9crattler.xe2x80x9d These localized rattlers can, in some cases, dramatically lower the phonon thermal conductivity (xcexalattice) to values comparable to that of a glass of the same composition. The theoretical lower limit of xcexalattice is designated xcexamin and corresponds to the thermal conductivity of an amorphous solid in which the mean-free path of the heat-carrying phonons approaches the order of the phonon wavelength. In an electrically conducting solid, heat is transported by both the charge carriers (electrons or holes) and phonons (lattice); hence, xcexa=xcexaelectron+xcexalattice. Therefore, a significantly enhanced ZT is expected from a minimized xcexalattice.
One class of materials that satisfies many of the requirements of an electron-crystal and phonon-glass solid is the filled skutterudites. Filled skutterudites have the general formula RM4X12, where typically: X is P, As, or Sb; M is Fe, Ru, or Os; and R is La, Ce, Pr, Nd, or Eu. Skutterudites are body-centered, cubic crystals with 34 atoms in the conventional unit cell and a space group of Im3. This structure consists of square, planar rings of four pnicogen atoms (i.e., X from the general formula) with rings of four oriented along either the (100), (010), or (001) crystallographic directions. The metal atoms M form a simple cubic sublattice, and the rattler atoms R are positioned in the two remaining holes in the unit cell. The structure of filled skutterudites differs from basic skutterudites in that basic skutterudites do not contain a rattler atom R. It has previously been shown that the xcexalattice of filled skutterudite antimonides is nearly an order of magnitude lower than that of the basic skutterudites due to the presence of the rattling atoms R.
Filled skutterudites are difficult to synthesize in pure form by conventional arc melting processes. Moreover, the observed low xcexalattice of these filled skutterudite antimonides is still significantly higher than the predicted xcexamin. There are a few possible sources for this problem, one of which is the presence of impurity phases such as RX2 or MX2. Another possible source may be that the filled skutterudites generated thus far are not actually similar to phonon glasses. Thus, the R elements used likely do not have sufficiently large anharmonic deflection to allow the skutterudites to approach the phonon glass limit.
The present invention overcomes the problems outlined above and provides new methods for the production of desirable high Tc and Jc (both magnetic and transport) value Hg-containing film superconductors having significantly improved epitaxial characteristics. As is conventional in the art, transport Jc refers to the measure of current density through a film when a current is directly applied to the film whereas magnetic Jc refers to the measure of current density through a film that is induced by the application of a magnetic field to the film. The present invention also provides methods for the production of thermoelectric materials having high ZT""s and low thermal conductivities. As is conventional in the art, ZT=TS2/xcexaxcfx81, where T is the maximum operating temperature of the material, S is the Seebeck coefficient, xcfx81 is the resistivity, and xcexa is the thermal conductivity of the material.
Generally speaking, the processes of forming superconductor films involves initial production of Tl-based superconducting films using known Tl-vapor annealing techniques, followed by Hg-vapor annealing to replace at least a portion of the Tl by Hg so that the desired Hg-based superconducting films are obtained. Tl-based superconducting films have similar crystalline structures to Hg-based superconducting films, but the Tc and especially Jc values at high temperature above 77K of the Tl-films are much lower. However, Tl is much less volatile than Hg and has a higher sticking coefficient. These factors make it much easier to grow Tl-based superconducting films that have greatly superior epitaxial properties (i.e., the grains are more uniformly aligned and are not randomly positioned on the substrate) compared to prior art films. When such precursor films are placed in an Hg-vapor environment and heated to elevated temperatures, Tl is driven out of the starting films and replaced by Hg simultaneously. Using appropriate time/temperature annealing conditions, it has been found that this exchange can occur without significantly altering the overall crystalline structure, so that high quality epitaxial Hg-based superconducting films can be obtained.
Thus, the methods of the invention broadly involve first providing a body made up of a substrate having a Tl-based superconducting film supported on a surface thereof, followed by annealing the body in the presence of Hg vapor and under conditions whereby at least a portion of the Tl of the starting superconducting film is replaced by Hg. Generally, this Hg-vapor annealing is carried out at a temperature of from about 600-900xc2x0 C. for a period of from about 1-20 hours, and more preferably at a temperature of from about 640-800xc2x0 C. for a period of from about 2-15 hours.
In applications where it is desirable to form micro-bridges, the Tl-based superconducting film is patterned and etched using conventional photolithography and etching techniques (i.e., a photoresist composition is applied to the film, exposed to activating radiation, developed, and etched). The remaining Tl film and newly formed micro-bridges are then annealed in the presence of Hg vapor as described above. The width of the micro-bridges is generally from about 2-10 xcexcm.
The most desirable Tl-based starting superconducting films for making Hg-1212 superconducting films are selected from the group consisting of Tl-1212 and Tl-2212 films (i.e., TlBa2CaCu2O7 and Tl2Ba2CaCu2O8), preferably yielding HgBa2CaCu2Ox films wherein x ran from about 5.8-6.2. The most desirable Tl -based starting superconducting films for making Hg-2223 superconducting films are selected from the group consisting of Tl-1223 and T1-2223 (i.e., TlBa2Ca2Cu3O9 and Tl2Ba2Ca2Cu3O10.5), preferably yielding HgBa2Ca2Cu3Ox films wherein x ranges from about 7.8-8.2. These are generally formed on a substrate, usually a single crystal substrate such as LaAlO3, by known techniques to form a highly epitaxial film.
The Hg-vapor annealing step can be carried out in a number of different ways. FIG. 1 sets forth a number of different orientations of the starting Tl-film bodies (shown as rectangles) with respect to a superconducting Hg-containing pellet and optionally in the presence of a Baxe2x80x94Caxe2x80x94Cuxe2x80x94O oxide pellet. It will be understood that these various annealing configurations are in practice preferably placed within a quartz tube (not shown in FIG. 1) whereupon a vacuum (on the order of from about 10xe2x88x924 to 1 Torr) is drawn and the tube is sealed. This sealed tube is then placed within the annealing furnace.
The final Hg-containing films are preferably either Hg-1212 or Hg-1223 films and usually have a thickness of from about 0.005-500 xcexcm and more preferably from about 0.1-1 xcexcm. In addition, the final Hg-containing films and micro-bridges formed of these films have very high transport and magnetic Jc values of at least about 106 A/cm2, preferably at least about 2xc3x97106 A/cm2, and more preferably at least about 2.3xc3x97106 A/cm2 at 100K and zero magnetic field and Tc values of from about 112-125K for Hg-1212 films (as used herein, Tc values refer to Tc (R=0), rather than onset Tconset values). These Jc values are quite significant because the prior art Hg-containing films, and particularly Hg-1212 films, have only been able to achieve Jc values of 106 A/cm2 at 77K and zero magnetic field. Furthermore, these prior art Jc values are magnetic Jc values and not transport Jc values. Because Jc depends on both the superconductivity of the molecule applied to the substrate as well as the manner in which the molecule grains are positioned on the substrate (i.e., the epitaxy), the current densities of films at higher temperatures are unpredictable based upon the current densities of the films at lower temperatures. The highly epitaxial character of the Hg-containing films of the invention is confirmed by the x-ray pole figures and Xmin values thereof which are up to about 50%, and more preferably from about 10-40%. Finally, Hg-1212 films according to the invention have a low microwave surface resistance of less than about 0.4 mxcexa9 and preferably less than about 0.3 mxcexa9 at 120K, values that have never been obtained above 100K in prior art superconductors.
The processes of forming thermoelectric materials according to the invention comprises providing, in a reaction vessel, a three-dimensional, crystalline precursor comprising a plurality of atoms and at least one molecule weakly bonded thereto. As used herein, molecule is intended to include elements as well as ions derived from those elements. For example, xe2x80x9clanthanum moleculexe2x80x9d is intended to include elemental lanthanum as well as lanthanum ions.
The precursor is then perturbed, causing the bond(s) between the molecule and the atoms to break, thus releasing the molecule from the precursor. The perturbing step can be accomplished by any method which subjects the crystalline precursor to energy of perturbation. Suitable methods include heat, light, ion beams, etc. When heat is utilized, the temperature to which the precursor is heated should be at least about 500xc2x0 C., and preferably at least about 1000xc2x0 C., depending upon the particular molecule which is weakly bonded to the precursor.
Either simultaneous to or shortly after the perturbing step, vapor under a controlled pressure and comprising a second molecule is introduced into the reaction vessel so that the second molecule replaces the released first molecule within the precursor. Preferably, the remaining crystalline structure of the precursor (i.e., structure other than the first molecule) remains essentially unaltered during this process.
Generally, any crystalline structure can be utilized as the crystalline precursor in the inventive processes, with body-centered crystals, orthorhombic crystals (such as perovskites), and cubic crystals being particularly preferred. One suitable type of body-centered crystal is the skutterudite having the general formula RM4X12, where M is a metal atom and R is a xe2x80x9crattler.xe2x80x9d Preferred metal atoms M are Fe, Ru, and Os, while preferred rattlers R (for use as the first molecule) are selected from the group consisting of La, Ce, Pr, Nd, and Eu. xe2x80x9cXxe2x80x9d in the general formula RM4X12 is preferably a pnicogen atom (e.g., P, As, Sb).
Any volatile molecule that is desired in the precursor at the same location as the first molecule can be utilized as the second molecule. Again, the same definition of molecule discussed above is intended to apply with respect to the second molecule. Thus, if a vapor of Hg is utilized, that vapor could comprise elemental Hg or Hg ions. Examples of suitable candidates for second molecules include those selected from the group consisting of Pb, Hg, Sn, In, Tl, and Ga. Because the second molecule is more volatile than the first molecule, it will have increased xe2x80x9crattlingxe2x80x9d abilities compared to the first molecule (i.e., increased local anharmonic vibrations around the rattler site). Thus, due to the presence of the volatile second molecule, the resulting thermoelectric materials have a phonon thermal conductivity (xcexalattice) at room temperature of less than about 0.012 W/cm-K, preferably less than about 0.01 W/cm-K, and more preferably less than about 0.007 W/cm-K. This low xcexalattice contributes to thermoelectric materials having a ZT of at least about 2, preferably at least about 3, and more preferably at least about 3.5.